A large number of photoelectric conversion devices used for detecting light are known. For example, visible light sensors or the like have been put into practical use. A large number of photoelectric conversion devices are used for devices which need illuminance adjustment, on/off control, or the like depending on human living environment.
In some display devices, ambient brightness of the display devices is detected so that display luminance is adjusted. This is because visibility can be improved and wasted power consumption can be reduced by detecting ambient brightness with photoelectric conversion devices so that appropriate display luminance is obtained. For example, examples of display devices having photoelectric conversion devices for adjusting luminance are mobile phones and computers having display portions.
A photoelectric conversion device includes a photoelectric conversion element such as a photodiode for a light sensing portion and can detect illuminance based on the amount of current flowing to the photoelectric conversion element. Reference 1 discloses a structure where current flowing from a photodiode is converted into a ramp-wave signal whose frequency is changed in accordance with the amount of the current and the converted ramp-wave signal is output as a digital signal.